Competition between pi donation and alpha-C-H agostic interactions in complexes of the type Tp'Ta(=CH-t-Bu)(X)(Y) (X equals halide; Y equals halide, NR(2), OR; Tp' equals hydrotris(3,5-dimethylpyrazolyl)borate)

The compounds Tp'Ta(CHC(CH3)(3))X(2) (2 (X = Cl), 3 (X = Br)) (Tp' = hydrotris(3,5-dimethylpyrazolyl)borate), have been synthesized by the reaction of KTp' with Ta(CHC-(CH3)(3))X(3)(thf)(2) in THF. One Cl ligand in 2 can be substituted with OR(-) (R = i-Pr, Me) or NMe(2)(-). The values of (1)J(C-H) for the alkylidene carbon are uniformly low due to the(alpha-H agostic interaction between the metal center and the alkylidene proton. The magnitude of. (1)J(C-H) in these compounds depends upon the pi donor properties of the remaining ligands. Attempted synthesis of monoarylamide complexes by displacement of Cl- from 2 with N(HAr- results in proton transfer from N to C and formation of the imido complexes Tp'Ta-(CH2C(CH3)(3))(NAr)Cl (Ar = 2,6-i-PrC6H3 (7), Ph(8)). When 2 reacts with KN(Me)Ph, proton transfer occurs from the Me group to the alkylidene carbon giving the imine complex Tp'Ta-(eta(2)-CH2=NPh)(CH(2)CMe(3))Cl (9). A single-crystal X-ray diffraction study of 9 confirmed the structure and suggested that 9 is best described as an azametallacyclopropane complex.